Pharmacokinetics is the study of the fate of pharmaceuticals and other biologically active compounds from the time they are introduced into the body until they are eliminated. For example, the sequence of events for an oral drug can include absorption through the various mucosal surfaces, distribution via the blood stream to various tissues, biotransformation in the liver and other tissues, action at the target site, and elimination of drug or metabolites in urine or bile. Pharmacokinetics provides a rational means of approaching the metabolism of a compound in a biological system. For reviews of pharmacokinetic equations and models, see, for example, Poulin and Theil (2000) J Pharm Sci. 89 (1):16-35; Slob et al. (1997) Crit Rev Toxicol. 27 (3):261-72; Haddad et al. (1996) Toxicol Lett. 85 (2):113-26; Hoang (1995) Toxicol Lett. 79 (1-3):99-106; Knaak et al. (1995) Toxicol Lett. 79 (1-3):87-98; and Ball and Schwartz (1994) Comput Biol Med. 24 (4):269-76.
One of the fundamental challenges researchers face in drug, environmental, nutritional, consumer product safety, and toxicology studies is the extrapolation of metabolic data and risk assessment from in vitro cell culture assays to animals. Although some conclusions can be drawn with the application of appropriate pharmacokinetic principles, there are still substantial limitations. One concern is that current screening assays utilize cells under conditions that do not replicate their function in their natural setting. The circulatory flow, interaction with other tissues, and other parameters associated with a physiological response are not found in standard tissue culture formats. For example, in a macroscale cell culture analog (CCA) system, cells are grown at the bottom of chambers. These systems have non-physiological high liquid-to-cell ratios, and have an unrealistic ratio of cell types (e.g., ratio of liver to lung cells). In a variant form of the macroscale CCA system the cells are grown on microcarrier beads. These systems more closely resemble physiological conditions, but are still deficient because they do not mimic physiological conditions accurately enough for predictive studies. Therefore, the resulting assay data is not based on the pattern of drug or toxin exposure that would be found in an animal.
Within living beings, concentration, time and metabolism interact to influence the intensity and duration of a pharmacologic or toxic response. For example, in vivo the presence of liver function strongly affects drug metabolism and bioavailability. Elimination of an active drug by the liver occurs by biotransformation and excretion. Biotransformation reactions include reactions catalyzed by the cytochrome P450 enzymes, which transform many chemically diverse drugs. A second biotransformation phase can add a hydrophilic group, such as glutathione, glucuronic acid or sulfate, to increase water solubility and speed elimination through the kidneys.
While biotransformation can be beneficial, it may also have undesirable consequences. Toxicity results from a complex interaction between a compound and the organism. During the process of biotransformation, the resulting metabolite can be more toxic than the parent compound. The single-cell assays used by many for toxicity screening miss these complex inter-cellular and inter-tissue effects.
Consequently, accurate prediction of human responsiveness to potential pharmaceuticals is difficult, often unreliable, and invariably expensive. Traditional methods of predicting human response utilize surrogates—typically either static, homogeneous in vitro cell culture assays or in vivo animal studies. In vitro cell culture assays are of limited value because they do not accurately mimic the complex environment a drug candidate is subjected to within a human and thus cannot accurately predict human risk. Similarly, while in vivo animal testing can account for these complex inter-cellular and inter-tissue effects not observable from in vitro cell-based assays, in vivo animal studies are extremely expensive, labor-intensive, time consuming, and often the results are of doubtful relevance when correlating human risk.
U.S. Pat. No. 5,612,188 issued to Shuler et al. describes a multicompartmental cell culture system. This culture system uses large components, such as culture chambers, sensors, and pumps, which require the use of large quantities of culture media, cells and test compounds. This system is very expensive to operate and requires a large amount of space in which to operate. Because this system is on such a large scale, the physiological parameters vary considerably from those found in an in vivo situation. It is impossible to accurately generate physiologically realistic conditions at such a large scale.
The development of microscale screening assays and devices that can provide better, faster and more efficient prediction of in vivo toxicity and clinical drug performance is of great interest in a number of fields, and is addressed in the present invention. Such a microscale device would accurately produce physiologically realistic parameters and would more closely model the desired in vivo system being tested.